1. Field of the Invention
The present invention relates generally to semiconductor processing equipment and methods. More particularly, the invention relates to a method and apparatus for retaining a semiconductor wafer on a surface during processing operations, such as plasma etching.
Semiconductor processing operations, such as plasma etching, ion implantation, chemical vapor deposition, and the like, frequently require that individual semiconductor wafers be retained on a flat or crowned surface, with a uniform and intimate contact being maintained between the wafer and the surface. For example, plasma etching requires that individual wafers be mounted on the exposed surface of an electrode located within a processing vessel. By positioning a second electrode proximate the exposed surface of the wafer and applying radio frequency energy to the electrodes, reactive plasma species are generated from introduced gases which will etch the surface according to well-known principles.
In order to achieve and maintain uniform etch characteristics, including directionality, selectivity, etch rate, and the like, it is necessary that the electrical and thermal environment proximate the semiconductor wafer be precisely controlled. In particular, it is necessary that the semiconductor wafer be tightly and uniformly held in place against the adjacent electrode surface in order to provide uniform cooling of the wafer and to minimize variations in the electric field produced by the pair of electrodes. Any variations in the thermal profile of the wafer or in the characteristics of the electric field will result in a non-uniform etch which can reduce both the quality and yield of the integrated circuits being produced.
In order to provide uniform contact between an individual semiconductor wafer and an adjacent surface, various clamping systems have been proposed for use in plasma etchers and other semiconductor processing equipment. Such previous clamping systems have, however, suffered from numerous drawbacks which result in less than uniform contact between the semiconductor wafer and the adjacent surface. For example, the clamping mechanisms seldom provide for precise positioning of the clamping element over the semiconductor wafer. The resulting misalignment of the wafer and the clamp frequently results in a varying contact pressure around the periphery of the wafer. The varying pressure, in turn, affects the rate of heat transfer from the wafer, resulting in undesired temperature gradients across the wafer. Thus, even very minor misalignment can substantially contribute to a non-uniform etch of the wafer. In addition to problems associated with misalignment, previous clamping mechanisms have frequently included components located too closely to the electrodes and thus have directly contributed to the generation of non-uniform electric fields. Moreover, the clamping mechanisms frequently cover a substantial portion of the peripheral region of the semiconductor wafer, thus reducing the yield of product from each wafer. For example, the clamping mechanisms have frequently been sized to extend radially inward a distance sufficient to cover the peripheral gaps created by the edge flats found on semiconductor wafers. Such additional overhang greatly increases the area of the wafer which is masked during the etching operation. In addition, prior art clamping systems have generally been unsuitable for use with variable gap plasma etchers, and have frequently resulted in the generation of relatively large quantities of particulates. Such particulates can adversely affect the fabrication process when they enter into the reaction chamber.
It would thus be desirable to provide for a wafer clamping mechanism which overcomes the problems just set forth.
2. Description of the Background Art
U.S. Pat. No. 4,340,462 describes a plasma reactor having parallel, spaced-apart electrodes where the electrodes are mounted on separate housings which are movable relative to each other to allow adjustment of the spacing between the electrodes. No clamping mechanism is provided for holding a wafer in place on the lower electrode. U.S. Pat. No. 4,282,924 discloses an ion implantation apparatus. A semiconductor wafer is held on a platen by a clamping ring mounted on the platen itself. U.S. Pat. No. 4,508,161 describes a method for holding a semiconductor wafer on a domed platen wherein the wafer is held about its periphery and a heat-conductive gas is introduced to the interstitial space between the wafer and the platen. U.S. Pat. No. 4,603,466 describes a domed platen for holding a semiconductor wafer where the wafer is held by a circular clamp.